1. Field of the Invention
The invention relates to power conversion including direct current (DC) to alternating current (AC) and, more particularly, to photovoltaic module output power conversion to AC.
2. Description of the Related Art
In coming years, distributed generation of electricity is likely to become a larger and larger part of the energy sourced to utility grids. Distributed sources of electrical energy such as solar photovoltaic modules, batteries, fuel cells and others generate direct current (DC) power, which must be converted to alternating current (AC) power for transmission and usage in residential and commercial settings.
Also, as distributed generation increases, the utility grid, commonly known as the “grid,” will be transformed to a still to be defined smart-grid that will support increased coordination between multiple generators and multiple loads. “Grid tied” photovoltaic systems are the most common form of solar electric systems today and they use a form of coordination called net-metering.
The larger number of photovoltaic installations are residential having an average capacity of about 2-3 kW. The bulk of new generating capacity is being installed in commercial buildings and utility scale installations. Residential systems commonly utilize single phase AC, while commercial systems most often use three phase AC.
Residential rooftops present a special challenge for the placement and interconnection of photovoltaic modules, due to the presence of gables, multiple roof angles, and other such obstructions. Such rooftops often do not expose a sufficiently large, commonly directed surface to the sun for photovoltaic modules to be positioned to harvest maximum power. Currently, conventional inverter-based interconnections are optimized to minimize IR (current times resistance) loss. This is referred to as string design. The inverters perform a function called maximum power point tracking (MPPT) on strings of PV modules. The MPPT process evaluates the PV module string output current-voltage curve on a continuous or sampled basis to determine the correct load voltage thus maximizing the string output power calculated as the string output voltage times current. Due to the nature of residential rooftops, the string design results in MPPT performance at the levels of the least power producing modules in the photovoltaic (PV) array. This degrades the AC power harvest from the entire array.
The use of microinverters in a one-to-one configuration with the PV modules removes the string design challenge, thereby enabling each PV module to produce current at its full capacity and truly permits MPPT at a per PV module level. The one-to-one arrangement of microinverters and PV modules is also referred to as AC PV modules in related art.
Commercial buildings and larger installations present slightly different challenges. In commercial buildings, large, commonly directed surfaces are generally available, but even then, obstructions, such as HVAC components must be dealt with as the components may block solar radiation. String design and MPPT also continue to be of concern. Additionally, since such installations often consist of thousands of PV modules, monitoring, operation and maintenance can be time consuming and expensive.
The use of AC PV modules for commercial installations simplifies string design, improves AC power harvest and provides the ability to remotely monitor the entire PV array on a module by module basis. A multiphase microinverter has the additional advantage of delivering substantially balanced multiphase AC power.
The benefits of microinverters have been documented in related art dating back almost three decades. Yet, the use of microinverters continues to be negligible due to their inferior reliability and efficiency as well as their high cost as compared to conventional inverters.
Typically, PV modules are placed in hostile outdoor environments in order to gain maximum exposure to solar radiation. Microinverters must be placed in proximity to the PV modules to realize their full benefits. Conventional inverters are typically placed in more benign environments, often indoors, e.g., on a protected wall or in a utility closet.
When microinverters are placed in proximity to PV modules, the hostile outdoor environment exacerbates the design challenge for achieving high reliability, high efficiency and low cost. Similarly, servicing and replacing microinverters on a rooftop is potentially more challenging and labor intensive than servicing and replacing centralized inverters.
The related art design approach for microinverters has been to implement them as miniaturized versions of conventional inverters, incorporating all the functions and components that were used in conventional inverters. Early versions of related art for microinverters utilized electrolytic capacitors, having a lifespan susceptible to degradation at high temperatures. Other versions of related art microinverters eliminate the electrolytic capacitor, thereby improving the lifespan.
FIG. 1 shows a simplified diagram of a related art grid tied photovoltaic system utilizing a conventional inverter. Referring to FIG. 1, PV modules 102 are mounted outdoors 110 for direct access to solar radiation and connected to a conventional inverter 105 using DC wiring 104. Both inverter 105 and DC wiring 104 are located in a weather protected region 111 such as the interior of a structure. The inverter 105 output feeds local AC loads 106 through AC wiring 103. The inverter's output is also tied for bi-directional flow of energy for net-metering to the utility grid 101 through exterior AC wiring 107.
FIG. 2 shows a simplified diagram of a related art grid tied photovoltaic system including microinverters. Referring to FIG. 2, PV modules 202 are mounted outdoors 210 for direct access to solar radiation. Microinverters 203 are electrically coupled in one-to-one proximity to the PV modules 202 (typically under them) and convert individual PV module DC outputs to AC power which is then fed to AC wiring 204. The AC wiring 204 feeds local loads 206 and the utility grid 201.
A problem with the related art microinverters is that they either collocate all inverter functions including safety and code compliance within the microinverter or they do not address how these functions are to be implemented, thereby making the design for high reliability and long life difficult and expensive. The collocation may also require redevelopment and replacement of the microinverter when code compliance requirements change.